Conditioning of wood to stabilise its colour

ABSTRACT

THE INVENTION RELATES TO A PROCESS FOR IMPARTING STABILIZED COLOUR TO WOOD. IN THE PROCESS, THE WOOD IS TREATED WITH A PERSULFATE OXIDISING AGENT SO AS TO PRODUCE THEREIN LIGHT-STABLE PRODUCTS WHICH STABILISE THE COLOUR OF THE WOOD.

United States Patent 3,690,922 CONDITIONING OF WOOD TO STABILISE ITSCOLOUR Stephen Dombay, Stevenage, England, assignor to FurnitureIndustry Research Association, Stevenage, England No Drawing.Continuation-impart of application Ser. No. 687,464, Dec. 4, 1967. Thisapplication Sept. 1, 1970, Ser. No. 68,781 Claims priority, applicationGreat Britain, Dec. 8, 1966, 55,026/66 The portion of the term of thepatent subsequent to Jan. 12, 1988, has been disclaimed Int. Cl. B27k5/02 US. Cl. 117-57 19 Claims ABSTRACT OF THE DISCLOSURE The inventionrelates to a process for imparting stabilized colour to wood. In theprocess, the wood is treated with a persulfate oxidising agent so as toproduce therein light-stable products which stabilise the colour of thewood.

This is a continuation-in-part application of my copending applicationSer. No. 687,464 filed on Dec. 4,

1967, now Pat. No. 3,554,785.

The invention relates to a method of treating wood so as to produce astable colour resistant to changes due to photochemical action.

It has been found that many woods change colour on exposure to light andthat this is due to photochemical activity. The change in colour isparticularly rapid with freshly-exposed surfaces.

Attempts have been made in the past to overcome this problem by applyingreducing agents to the woods or by reducing the effect of light byincorporating ultra-violet absorbers therein. It has now surprisinglybeen found that it is possible to achieve a more satisfactory resultthan in the past by a particular oxidation of ligneous ingredients. Bymeans of the invention a stabilised colour may be imparted to thematerial, so that for example only slight or imperceptible changes takeplace in that colour even after quite long exposure to light. Theinvention is particularly applicable to solid wood or wood in the formof a veneer.

The invention provides a method for producing stable colour in woodmaterial which comprises treating the material with a persulfateoxidising agent so as to oxidise natural substances present in thematerial in a particular way whereby there are produced oxidationproducts which are substantially stable to light, so that the colour ofthe material containing such oxidation products is stabilized.

The material to which the invention is especially applicable ismahogany, sapele, walnut, afrormosia, munin= ga, utile, iroko, tola,oak, cherry and maple woods. The term walnut" covers all walnuts,including European, American and African walnuts and pecan, and the termmahogany covers all mahoganies, including American and Africanmahoganies.

The oxidising agent may of course be a mixture of oxidising agents. Asindicated above, the persulphate oxidising agent produces relativelylight-stable oxidation prodnets in the material. The use of apersulphate is crucial in yielding the improved stable colour.Persulphates (for example ammonium or alkali metal persulphate) eitheralone or in combination with other agents such as hypochlorites orperoxides (added with the persulphate's or separately therefrom) havebeen found particularly satis= factory in producing a good stablecolour. Hypochlorous acid (for example with a pH of 2 to 7) is in somecases very suitable as the other agent, or the other agent may beobtained by adding free acid to an alkaline solution of 3,690,922Patented Sept. 12, 1972 hypochlorite. If a hypochlorite is used, it maybe an alkaline solution containing at least 0.1% of available chlorine.

The persulphate is used normally as a solution containing from 10 g. perlitre up to the solubility limit thereof.

Chlorates and perchlorates have been found unsuitable because the colourproduced is less stable; nitrates may give an inadequately stable colourfor some purposes. Other contemplated oxidising agents include iodates,periodates, bromates and perbromates, but these are rather expensive.None of these gives the remarkable and sur prising effects given bypersulphates.

The oxidising agent is normally applied to the material, but it maysometimes be found convenient to produce it in situ in the material, forexample by applying to the material substances which will react to formthe oxidising agent. a

The oxidising agent may be applied to the material in the form of asolution thereof. The material is preferably completely immersed in thesolution, or treated therewith by swabbing or the like.

The oxidation must, of course, be a fairly gentle one so as not todestroy the nature of the material, but it may be sufiiciently strong sothat the oxidation products have a colour somewhat different from thecolour of the ma-= terial before treatment.

This confers a further advantage on the invention in that it can be usedfor the additional purpose of providing the treated material with agreater uniformity of colour than it had before treatment. By this meansdifferent samples of wood within the same species may be given moresimilar colours and this facilitates matching of materials of the samespecies from different sources.

This advantage is of particular value in the furniture industry. Afurniture factory frequently has to get its supplies of wood of onespecies from a number of different sources. It would often happen thatpieces of wood from different sources would be brought together in asingle piece of furniture, but hitherto this has often had to be avoidedbecause the different pieces of wood, although of the same species, haveoften had different colours, so that a piece of furniture containingthem would be commercially unacceptable. By means of the presentinvention, this source of inefficiency in furniture manufacture can beavoided, as the wood from the various sources can be treated by thepresent method so as to be more uniform in colour.

A further advantage is that the method of the invention can be used toconfer greater uniformity of colour in a single piece of wood. Thus, itsometimes occurs that exposure of part of the surface of a piece of woodto light causes localised discolouration or darkening, leaving a patchyappearance. Treatment of the surface by the present method can bring thevariously coloured portions into greater colour uniformity.

It is, of course, not necessary to treat the full thickness of a pieceof the material of which only the surface is to be seen in use. It hasbeen found generally adequate in practice to treat the wood to a depthof up to 1 mm. and a stabilised layer of this thickness is frequentlymore than adequate.

It is frequently found desirable to use an oxidation catalyst (forexample one containing permanganate or cupric ions) so as to acceleratethe oxidation, and thus save time in carrying out the process. Theoxidation catalyst may fog xample be applied as a single solution withthe oxidising agent, or as a separate solution.

The most suitable auxiliary oxidising agent (if any), solvent, pHconcentration, time of application and the like may vary from wood towood and may partly depend on the particular final colour and thethickness of the stabilised layer which is desired. The most suitableparameters can in any particular case be found by tests involvingvariation of the parameters indicated above.

An important preferred feature of the invention is the control of the pHduring the oxidation. It has been found that the colour and itsstability may partly depend on the pH. Frequently it is very desirableto apply pH adjusting agents to the material so as to impart a suitablepH during treatment; generally, an alkaline pH is advantageous.

Of course, the treatment according to the invention may be followed by aneutralising step so as to give the treated material a final pH which issubstantially neutral. Thus, if the only agent used is an alkalinepersulphate solution, this may be followed by an acid wash to neutralisethe alkali remaining in the material.

The pH adjusting agent may for example be as a single solution with theoxidising agent, or as a separate solution. Many oxidising agents (suchas persulphates) have been found to form acid reaction products whenapplied in the present process; if it is found that these acid products,particularly if added to the natural acidity of many woods, cause toolow a pH, then a suitable alkaline agent may be applied to the material.Sometimes the required pH may be maintained by means of a pH buffer, forexample a phosphate or acetate buffer.

It is generally convenient to use the oxidising agent in the form of asolution containing a pH adjusting agent to impart a predetermined pH(optionally with at least 0.1 g. of an oxidation catalyst per litre),the remainder of the solution consisting essentially of water with orwithout a lower alcohol and optionally containing a surface-activeagent.

Where the treatment imparts an acid or alkaline pH to the material, itmay be desirable to bring the pH to a moderate value, say -8, by aftertreatment of the material with an alkaline or acid material, asindicated above.

The single oxidising step may leave a surface layer of wood which is notonly stabilised in colour but which has an acceptable colour. This hasbeen found to be the case with, for example, African mahogany andEuropean walnut.

In many cases, it has been found desirable to effect the method instages, the first stage being as described above and the second stagebeing a second treatment with a second oxidising agent so as to modifythe stabilised colour (which may reinforce stabilisation), this has beenfound desirable in, for example, the cases of afrormosia and Africanwalnut. Hypochlorite and hydrogen peroxide are examples of secondoxidising agents which have been found beneficial in certain cases. Forexample, medium dark stabilised shades of colour have successfully beenproduced in African walnut and in afrormosia by using a persulphate asfirst oxidising agent and a hypochlorite as a second oxidising agent.

A final stage of washing the material with a liquid of pH other than 7is a desirable optional feature of the method of the invention for thepurpose of neutralising any excess acidity or alkalinity of the surfaceof the material and giving it a pH of 5-8. This liquid can also act as awash to remove any unwanted solid substance on the surface of thematerial.

After treatment by the method of the invention, the material may bedried in air or force-dried, for example by a current of heated air.Some time is desirably allowed to elapse between the treatment and theforce-drying; the minimum time varies from wood to wood, but a time ofat least 5 minutes is generally required for best results.

In addition to the above general teaching, the invention providesspecific guidance to cover most practical case's. In particular, thefollowing ranges of compositions are an important feature of theinvention.

While a treatment with the persulphate only is very advantageous ingiving a stable colour, it is possible to help in controlling the actualshade of colour and in giving optimum results in some cases by usingadditional steps and carrying out the process in stages.

For the first stage of the method of the invention, it is preferred touse as oxidising agent, particularly for dark or medium coloured woodssuch as 'sapele (Entandrophragma cylindricum African mahogany (Khayaivorensis), afrormosia (Afrormosia elata), muninga (Pter0 carpusangolensis), utile (Entandrophragma utile), iroko (Chlorophora excelsa),tola (Oxystimga oxyphyllum), African walnut (Lovoa klaineana), andEuropean walnut (Juglans regia), a solution containing from 10 g. perlitre up to the solubility limit of potassium, sodium or ammoniumpersulphate, the remainder of the solution consisting essentially ofwater and a lower alcohol and optionally a surface-active agent (such as5-15 ml. of Teepol, a surfactant based on the sodium salts of secondaryalkyl sulfates, cf. Kingzetts Chemical Encyclopedia, 9th edition, p. 959or Lissapol, a surfactant, either sodium oleylsulfate (Lissapol-C) or anoctylcresol condensation product with ethylene oxide (Lissapol NX), cf.Hackhs Chemical Dictionary, 4th edition, p. 392). Such a solution isherein called solution A.

For light-coloured woods (such as oak or maple) it is sometimesadvantageous to precede the persulphate treatment by a bleaching step,for example with an alkaline hypochlorite solution containing 1-5% ofavailable chlorine, or a hydrogen peroxide solution.

For the second oxidising stage (if one is used), it is preferred eitherto use a solution containing, per litre, sodium hypochlorite in anamount equivalent to 500 ml. of a sodium hypochlorite solution of 10-12%available chlorine, and a pH adjusting agent in an amount to give thedesired pH, the remainder consisting essentially of water, or to use a10-100 volumes hydrogen peroxide solution which desirably contains 1-5%ammonium acetate by weight. Such a solution is herein called solution B.The sodium hypochlorite solution is generally used when a persulphatehas been used as the first or main oxidising agent that is, in manycases where solution A has been used). The hydrogen peroxide solution isgenerally used after solution A in the case of teak, and after solutionB in the case of light-coloured woods.

In its application to the material, the selected solution is in eachcase preferably swabbed on to the surface of the material forapproximately 5 minutes. Between successive stages a period of time ispreferably allowed for drying; this time may be reduced by force-drying,for example in a current of warm air. Desirably the material is allowedto remain at least five minutes at room temperature.

If the material to be treated is sapele, African mahogany, afrormosia,muninga, utile, iroko or tola, and the final colour is to be light, thenit may be treated in a first stage with a solution A (having an alkalinepH), in a second stage with a solution B (having an alkaline pH), and ina final stage with a solution of a weak acid (for example containing20-100 ml. of glacial acetic acid per litre) to restore the pH to 5-8.In this case preferred solutions A and B contain respectively thefollowing ingredients in approximately the following amounts per litre-A: 30 g. of potassium persulphate, 30 g. of sodium carbonate, 0.5 g. ofpotassium permanganate, ml. of methylated spirit, 10 ml. of of Teepol orLissapol, and water to make up a litre; B: 200 ml. of sodiumhypochlorite solution containing 10-12% available chlorine, 10 g. ofsodium carbonate, and water to make up a litre.

If the material to be treated is European walnut, or if it is sapele,African mahogany or utile, and the final colour is to be dark, then itmay be treated in a first stage with a solution A (having an alkalinepH) and in a second, final stage with a solution of a weak acid (forexample containing 20-100 ml. of glacial acetic acid per litre) torestore the pH to 6-8. In this case a preferred solution A contains thefollowing ingredients in approxi tion B (having an alkaline pH) and in athird stage with 10 a solution of a weak acid (for example containing20-100 ml. of glacial acetic acid per litre) to restore the pH to 6-8.In this case preferred solutions A and B contain respectively thefollowing ingredients in approximately the following amounts perlitre--A: 30 g. of ammonium persulphate, 100 ml. of 0.88 ammonia, 0.5 g.of potassium permanganate, 150 ml. of methylated spirit, 10 ml. ofTeepol or Lissapol, and water to make up a litre; B: 200 ml. of sodiumhypochlorite solution containing 10-12% available chlorine, 10 g. ofsodium carbonate and water to make up a litre.

It has been found that, in general, pink or light red woods may betreated with the same solutions as sapele and African mahogany, whilebeige or pale brown woods may be treated with the same solutions asutile, iroko and tola.

After treatment by the method of the invention, the material may besubjected to a conventional finishing operation. For example, wood maybe finished by de-nibbing or sanding after filling or sealing. Heavysanding is desirably avoided as it may remove the layer stabilised bythe method of the invention, if such layer (as is usual) is not morethan one millimetre thick.

When using the method of the invention to bring differently colouredsamples of the same species of wood into greater colour uniformity, itmay be advantageous to apply to the lighter coloured samples a solutionwhich has a darkening effect as well as a stablising effect, and to thedarker samples a solution which has a lightening effect on colour aswell as a stabilising effect. A solution B generally has a lighteningaffect and a solution A generally has a darkening efiect. Thesesolutions are suitably used at half the strengths indicated herein, andmay be followed by the treatment with acetic acid solution, the

latter being used at full strength. For example, in order to bringdifferent samples of afrormosia into greater colour uniformity, thedarker samples may be treated with a half-strength solution B and thelighter samples with a half-strength solution A, followed in both casesby treatment with acetic acid solution of the strength hereinbeforeindicated.

The invention also provides wood materials, either solid woods orveneers, treated by the method of the invention, as well as furnitureand other artifacts made of or containing such treated material.

It is important to note that the process of the invention not onlyproduces stabilised colours: it also enables different colours to beobtained by variation of the parameters mentioned above, such as the pH,the nature and strength of the oxidising agents, and the use of morethan one treatment with different agents.

The invention is illustrated by the following examples.

EXAMPLE 1SAPELE A solution A was used containing 50 g. of potassiumpersulphate, 50 g. of sodium carbonate, 0.5 g. of potassiumpermanganate, 150 ml. of methylated spirit, 10 ml. of Teepol and enoughwater to make up one litre. The solution was applied to a sample ofsapele by swabbing for about 2 minutes. The sapele was then allowed todry for about an hour. It was then swabbed for about 2 minutes with asolution B containing 200 ml. of a sodium hypochlorite solutioncontaining 10-12% of available chlorine, 10 g. of sodium carbonate andwater to make up a litre. The sapele was then allowed to dry for anhour. It was then swabbed for about 2 minutes with a solution of aceticacid containing 50 ml. of glacial acetic acid and water to make up alitre. It was then completely dried and coated with one layer of a clearnitrocellulose lacquer.

The treated sapele panel and a control (untreated) panel more exposed toa north light for eight weeks, their reflectance being measured beforeand after such exposure. The results are given in Table 1. The treatedpanel had a much more stable colour than the control panel.

EXAMPLE 2--AFRICAN WALNUT A panel of African walnut was treated asdescribed in Example 1 except that solution A contained 50 g. ofammonium persulphate and 100 ml. of 0.88 ammonia in stead of thepotassium persulphate and sodium carbonate.

The reflectance results obtained with the treated panel and a controlpanel are given in Table 1. The treated panel had much more stablecolour than the control panel.

EXAMPLE 3-AFRORMOSIA A panel of afrormosia was treated as described inExample 1.

The reflectance results obtained with the treated panel and a controlpanel are given in Table 1. The treated panel had a much more stablecolour than the control panel.

EXAMPLE 4EUROPEAN OAK A panel of European oak (Quercus robur) waspretreated being immersed for about 5 minutes in a solution contaiuing30 g. of potassium persulphate, 30 g. of sodium carbonate, 0.5 g. ofpotassium permanganate, 150 ml. of methylated spirit, 10 ml. ofsurface-active agent and water to make up a litre. It was allowed todry. It was then immersed for about 5 minutes in a solution A1 of sodiumhypochlorite containing 3% of available chloride and 2% of sodiumcarbonate. It was then dipped, while still wet, into a solution ofhydrogen peroxide of 40 volumes strength, with agitation. The wood wasfinally rinsed with water and dried.

TABLE 1 Reflectance values obtained with the FIRA Spectrophotometerusing nine Iltord colour filters Nos. 601-600. Readings were taken oftest panels and controls before and after an eight-week period ofexposure to the north light. The values in the table represent percentreflectance oi a chrome yellow surface, used as standard(reflectance=100%). The values for controls are given in parentheses.

Filter No.

P 8 transmission my. 426 470 490 520 650 580 600 660 685 T C T O T C T OT O T O T O T O T C Wood:

Entgtdrophragma cylmdricum (sapele):

A 71 (74) Afrormosz'a elata (afrcrmosia)' NOTE.B=before exposure,A=elter exposure, T=treated specimens, 0 =control.

3% acetic acid in water and dried completely at ambient temperature.

EXAMPLE 9 Species of wood and solutions used were as in Example 8, butinstead of leaving the samples at ambient temperature after applicationof solution A, they were exposed to a stream of warm air at 60 C, for 5minutes, and instead of the final drying at ambient temperature theywere dried completely at 60 Ca TABLE 2.REFLECT ANCE CHANGES FOR WHITEOAK Filters 1 2 3 4 5 6 8 8 9 Before exposure 76(97) 77(97) 5902 64(63)55(66) 59(67) 66(71) 80(86) 93(96) After exposure 79(87) 79(87) 60(64)55(61) 57(60) 60(61) saws 82(80) 93 90 EAMPLE 5 EXAMPLE 10 Samples ofthe followmlg Woods welie used: Samples of the following woods wereused: giant (European, American or African) Cherry Oak Sapele andAfrican walnut Cherry Maple Equal volumes of a solution A and a solutionB, the Mahogany compositions of which were as in Example 9, were mixedIroko together and the mixture was applied by swabbing for Sapele andabout 3 minutes, then the samples were left at ambient Makoretemperature for 15 minutes and washed with 3% acetic A solution A wasused containing 40 grammes of ammonium persulphate, 100 millilitres of0.88 ammonia, 150 ml. of methylated spirit, 10 ml. Teepol and enoughwater to make upon one litre. The solution was applied to samples of theabove species by swabbing for about 2 minutes, then the samples wereleft to dry completely at ambient temperature and coated with a clearnitrocellulose lacquer,

EXAMPLE 6 Samples of all the woods listed in Example 5 were used, Asolution A the composition of which was as that in Example 5 was appliedto the samples by swabbing for about 5 minutes, then the samples wereexposed to a stream of warm air of about 60 C. until completely dry andwere finally coated with a clear nitrocellulose lacquer,

EXAMPLE 7 Samples of all the woods listed in Example 5 were used, Asolution A the composition of which was as that in Example 5 was appliedby swabbing for about 2 minutes, then the samples were left to partlydry at ambient temperature for 20 minutes, then washed with 3% aceticacid in water and dried completely.

EXAMPLE 8 Samples of the following woods were used:

Iroko Pecan Sapele Cherry and Makore Maple Walnut acid in water, Thesamples were finally dried at ambient temperature.

EXAMPLE 11 Species, solutions and method as in Example 10, except thatthe final drying was carried out in a stream of warm air, approximately60 C,

EXAMPLE 12 Samples of oak were used.

A bleaching solution containing 15% (50 volumes) hydrogen peroxide inwater was first applied as a single swab and after an interval of 5minutes a second solution identical in composition with solution B inExample 8 was applied by swabbing for 2 minutes, then the samples wereleft to partly dry at ambient temperature for about 20 minutes andswabbed with a third solution containing 30 grammes of potassiumpersulphate, 20 grammes of so dium carbonate, ml, methylated spirit, 10ml. Teepol and enough water to make one litre and after an interval of15 minutes, the samples were washed with 3% acetic acid in water anddried completely,

In all of Examples 5 to 12, the resulting treated samples were found tobe much more stable in colour than central (untreated) samples of thesame woods,

What is claimed is:

1. A process for producing stable colour in a wood comprising subjectingwood selected from the group consisting of sapele, mahogany, walnut,oak, cherry, maple, afrormosia, muninga, utile, iroko and tola to atreatment with a persulfate oxidizing agent under alkaline conditionswhereby a stabilized colour is imparted to the wood.

2. A process according to claim 1 wherein the persulfate is selectedfrom the group consisting of potassium and ammonium persulfate.

3. A process according to claim 1 wherein, after the treatment with thepersulfate oxidizing agent, the pH of the wood surface is adjusted tofrom about 5 to about 8 with a weak acid,

4. A process according to claim 1 wherein the wood being treated is amahogany 5. A process according to claim 1 wherein the wood beingtreated is a walnut.

6. A process according to claim 1 wherein the wood being treated is oak.

7. A process according to claim 1 wherein the wood being treated ischerry.

8. A process according to claim 1 wherein the wood being treated ismaple.

9. A process for producing stable colour in a wood comprising subjectingwood selected from the group consisting of mahogany, walnut, oak, cherryand maple to a first treatment with a persulfate oxidizing agent underalkaline conditions, subjecting the wood to a second treat-= ment with ahypochlorite oxidizing agent under alkaline conditions and thenadjusting the pH to from about 5 to about 8 with a weak acid whereby astabilized colour is imparted to the wood.

10. A process according to claim 9 wherein the persulfate is selectedfrom the group consisting of potassium and ammonium persulfate.

11. A process according to claim 9 wherein the pH adjustment is elfectedwith acetic acid.

12. A process according to claim 9 wherein the per= sulfate oxidizingagent is catalyzed with a catalyst se-= lected from the group consistingof permanganate and cupric ions.

13. A process according to claim 9 wherein the wood being treated is amahogany.

14. A process according to claim 9 wherein the wood being treated is awalnut.

15. A process according to claim 9 wherein an aqueous potassiumpersulfate solution is employed containing per liter about 30 gms. ofpotassium sulfate, about 30 gms. of sodium carbonate, about 0.5 gm. ofpotassium permanganate, about 150 ml. of methylated spirit, about 10 ml.of a surface-active agent, and water to make up a liter, and thehypochlorite is added as an aqueous solution containing per liter about200 ml. of sodium hypochlorite solution containing about 10-12%available chlorine, and about 10 gms. of sodium carbonate.

16. A process according to claim 9 wherein the pH adjustment is elfectedwith an aqueous solution that contains from about 20 to about ml. ofglacial acetic acid per liter of solution.

17. A process according to claim 9 wherein the wood being treated ischerry.

18. A process according to claim 9 wherein the wood being treated isoak.

19. A process according to claim 9 wherein the wood being treated ismaple.

References Cited UNITED STATES PATENTS 2,096,400 10/ 1937 Kritchevsky 8111 3,348,903 10/1967 Baier 8-111 3,429,772 2/1969 Harvey et al. 8-111 XR3,554,785 1/1971 Dombay 117-57 MAYER WEINBLATI, Primary Examiner US. Cl.X.R.

